Natural gas, containing mainly methane, constitutes a large fraction of the world's fossil hydrocarbon resource but is today primarily used as fuel for heating and electricity generation. There is enormous economic potential in developing technologies that convert natural gas to more valuable chemicals. Well established commercial routes for conversion of methane exists e.g. through oxidative reaction paths with synthesis gas (a mixture of CO and H2) as an intermediary product that is then in a second reaction converted to products such as methanol, liquid fuels (by Fisher-Tropsch method) or ammonia (by Haber-Bosch method).
A somewhat less investigated route for natural gas conversion to valuable chemicals is through non-oxidative reactions. Avoiding the oxidation of the natural gas will improve the energy efficiency of the process and thus reduce CO2 emissions while potentially also improving economic return. A promising approach in this context is dehydroaromatization of methane as a method of selective methane conversion directly to aromatic hydrocarbons, mainly benzene, according to the overall reaction formula (I):6CH4→C6H6+9H2  (I)
The groundbreaking work on this approach was published in 1993 by Wang et al, Catal. Lett. 21 35 (1993) and involved a catalyst with molybdenum deposited on HZSM-5 zeolite support with reactions taking place at a temperature of 700° C. Since the publication by Wang et al, many academic groups and commercial companies have been working to improve the process, with some significant progress being made but still with some remaining barriers to large scale commercial deployment.
Non-oxidative dehydroaromatization of methane to benzene is thermodynamically limited, leading to two significant technological obstacles to scale-up and commercialization:
1) The fact that the equilibrium conversion of methane at typical reaction temperature 700° C. and pressure 1 bar is thermodynamically limited to about 12% aromatic product (M. Sohrabi et al., Chem. Eng. Technol. 2012, 35, No. 00, 1-9), with experimental conversion often reported to be slightly less at around 8-11%; and
2) The formation of several forms of carbon deposits in the catalyst bed, including polyaromatic hydrocarbons, leading to rapid deactivation of the catalyst and very short catalyst lifetime.
Several groups have proposed the use of hydrogen selective membranes for thermodynamic equilibrium shift as a solution to overcome obstacle 1) noted above. For example, Li et al. Chemical Engineering Science 57 (2002) 4595-4604 have laid out numerical models to support such an approach. Illuta et al. in Ind. Eng. Chem. Res. 2002, 41, 231-2378 have demonstrated use of a hydrogen permeable Pd-membrane to shift the equilibrium of the methane dehydroaromatization process. One problem with Pd-membranes is that they promote the undesirable side reactions 2) towards coke formation. WO2010/115671 describes a method to electrochemically remove hydrogen from a reaction mixture using an electrochemical membrane electrode assembly. In principle, this works similar to the hydrogen permeable membrane but using an applied bias instead of a chemical potential gradient through a partial pressure difference in pH2. Such a process will be subject to significant formation of polyaromatic hydrocarbons and coke.
The present inventors have proposed the use of a lanthanum tungstate based mixed metal oxide membrane as set out in WO2011/098525 to overcome the limitations of pre-existing membranes. However, the equilibrium shift over a membrane in itself does not solve the fundamental problem of coke formation and deactivation of the catalyst.
Thus whilst removal of hydrogen brings the equilibrium over towards the right hand side of the equation (I) above, it will be appreciated that the dehydrogenation catalyst employed also causes conversion of methane through to carbon itself and that conversion process is also encouraged by hydrogen removal.
Another approach to obstacle 1) above is to accept a low/moderate single pass conversion and focus the design of the process towards an efficient recycle of unreacted alkanes back into the reactor. This requires hydrogen removal from the recycle gas outside of the reaction zone. Coelho et al. describes such a process in WO2010/115768 (U.S. Pat. No. 8,487,152 B2), where hydrogen is electrochemically removed from the product stream outside of the reaction zone and then unreacted alkanes are recycled back into the reactor. The removal of hydrogen in an external gas loop does not in itself solve the problem of coke formation with subsequent deactivation of catalyst but the toleration of low yields helps reduce its formation. A process scheme that has been proposed by several groups to overcome obstacle 2) above is described in for example in US2009/0240093. The idea is to add an amount of hydrogen, water and/or CO2 to the feed gas for the purpose of suppressing coke formation and increase the catalyst time on stream. A somewhat different process proposed by US 2013/0090506 describes an alternative process where hydrogen pulses are used in the feed process.
The drawback of such a process is that co-feeding with hydrogen moves the equilibrium to the left and hence reduces the formation of aromatic hydrocarbons.
Another process scheme to overcome obstacle 2) above is to let the catalyst deactivate and have a continuous system for moving the catalyst between a compartment for dehydroaromatization and compartment for catalyst regeneration. One variant of this approach has been commercialized by UOP under the Cyclar trademark and is described for example by Fukanaga in Chemical Engineering (Tokyo), Vol. 44; No. 4; page 283-287(1999).
Another variant for continuous catalyst regeneration is a duel-zone fluidized bed, such as the design described by Scheinder et al in WO2012/022569. A drawback of continues movement of catalyst between different compartments is increased complexity and cost of the entire system.
Thus, there is a need to develop a new alkane dehydroaromatization process that represents an acceptable compromise between high yield conversion of light alkanes to aromatics and long catalyst life time.
The present inventors have realised that the dehydroaromatization of alkanes such as methane in the presence of a dehydrogenation catalyst can be improved by the addition of hydrogen as a feed to the reactor in combination with removal of hydrogen from the reactor and optionally in combination with a recycle of hydrogen and/or unreacted starting material. Furthermore, process stability might be improved by allowing a limited amount of oxygen transport through the membrane into the reactor zone.
By feeding hydrogen at the start of the process, the initial equilibrium in the reaction in scheme 1 above is moved to the left thus avoiding initial coke formation and maximising catalyst life. Thereafter, by removing hydrogen within the reactor, the thermodynamic equilibrium can be at least maintained or optionally subsequently shifted to the product side as the reactants pass through the reactor. This process allows a dynamic control over the equilibria within the process thus maximising yields whilst minimizing coking of the catalyst.
Thus, the reaction gas mixture, temperature, pressure etc may change as the reaction gases (and product gases) move through the reactor from the inlet to the outlet. A key benefit of the invention is that the combination of co-feed with hydrogen and use of the hydrogen membrane allows the operator to optimize the partial pressure of hydrogen dynamically throughout the length of the reactor, so as to maximize selectivity and minimize coke formation. An added benefit is the economical integration of a recycle loop for unreacted gases with no need for external hydrogen separation from the unreacted gas mix as the reactor system itself has in-situ hydrogen removal. Another added benefit is an increase in process stability due to a controlled oxygen transport into the reactor zone through the membrane, which decreases the coke formation.